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TECHNOLOGY: New Technique Identifies Plastics In Auto Recycling

Bill Siuru | Aug 01, 1995

The push for automobile recycling has intensified in recent years, both in the United States and abroad. Typically, salvage or junk yards recycle and sell the steel and iron components, which make up approximately 70 percent by weight of the average automobile.

However, recycling automotive plastics is more complicated since the different plastic types must be identified and sorted before recycling. Also, because different plastic types cannot be mixed to manufacture new components, a simple identification process is the key to cost-effective plastics recycling. Although most auto manufacturers currently mold identifying codes into plastic parts, these codes are often difficult to find and decipher, especially in old, worn-out or wrecked vehicles. In many cases, the part must be removed to find the code.

Ford of Europe, in conjunction with the University of Southampton in England, has developed an alternative method to identify plastics before they are stripped from scrapped vehicles. The Tribo-electric Identifier, or Tribo Pen, uses the "tribo effect," based on the principle that when two non-conducting materials are rubbed together, the friction between surfaces generates a distinct, measurable static charge. The value and polarity of the charge is different for each family of plastic materials, such as polypropylene (PP), polyethylene terephthalate (PET) and acrylonitrile butadiene styrene (ABS). By measuring the charge and comparing it against a table of reference values stored in the pen's memory, the unknown materials can be identified.

The hand-held pen (see diagram), which uses a series of different colored lights to identify materials, has three integral heads of reference materials with known positions in the tribo-electric series. The heads are composed of brass, PP and PET. When the device is rubbed against the clean, nearly charge-free surface of an unidentified plastic, the pen's three heads will generate electrostatic charges.

For instance, if the unknown material is rubbed with the brass or PP head and generates a positive charge, and then is rubbed with the PET head and generates a negative charge, the pen would indicate that the unknown material is ABS. This conclusion is reached by comparing measurements against the triboelectric series for brass, PP and PET in the pen's memory.

The pen can be used by relatively unskilled operators. So far, the device can distinguish between four main plastic families; Ford is working on doubling that number. Although the technology is being developed for use with plastic automotive components, the device and techniques reportedly could be used for other plastic products.